I can't make sense of this. Why would the auto brake activate when a rider/car approaches from behind?

I could maybe understand if there were cyclists at the side or being passed, but agree, it's a little counter intuitive.
_________________Find a happy place, find a happy place
http://mikemccrumpet.wordpress.com/

I can't make sense of this. Why would the auto brake activate when a rider/car approaches from behind?

It would've been the car in front of the car that braked that caused the problem, a driver would've gone round / not braked etc. but the autobraking was excessive.
_________________Jibbering Sports Stuff

This was the start of the race? Normally there is only one commissioner's car that the riders have to stay behind (until the flag is dropped)?

The video shows what it was that almost certainly caused the braking, the motorbikes... and there's a lot more than one comm car in front of the bunch, although it depends how far in front, lead car should of course be seconds ahead of the bunch in all cases.

Even in domestic #@?# level racing there's a lead car and comm 2 in front of the bunch at the roll out (and normally lead car 2 too, although that could be parked up.)
_________________Jibbering Sports Stuff

Looks the peloton was just starting to come round the lead car when the crash happened.

Hard to see... when it cuts to the copter shot the incident has pretty much already happened and the bikes are only just making it round the sides.

Possibly picked up one of the camera bikes coming through but they also look a fair way off. The one that's initially behind the tree with the camera held off the side perhaps ?
_________________Failures are finger posts on the road to achievement [C.S. Lewis]